1. Field of the Invention
The present invention relates to a liquid detecting apparatus or a liquid-amount detecting apparatus for detecting a liquid or the amount of a liquid in a container. For example, the present invention relates to an apparatus for detecting the remaining amount of ink in an ink tank of an ink-jet printer.
2. Description of the Related Art
In an ink-jet printer, ink is stored in an ink tank, and the ink is transferred from the ink tank to an ink discharging unit (head) through an ink path, whereby droplets of ink is discharged. In the ink-jet printer, the presence or absence of ink must be detected with a relatively high precision. A first reason for this is that it is difficult to visually determine the remaining amount of ink from the external appearance of the ink tank.
As a second reason, if the ink is discharged to such an extent that the ink is completely used up, ink that serves as “primer” becomes unavailable. That is, air enters the ink path, preventing the ink from being transferred to a nozzle. In that case, ink must be supplied again from the start to allow ink to be discharged, or the ink discharging unit could be degraded. As a method of discharging ink in an ink-jet printer, the thermal method is known, in which ink in an ink cell is rapidly heated by a heating element to discharge droplets of ink. If the heating element generates heat even though ink is not present, the heating element could be damaged. Thus, discharging of ink (printing) must be stopped when the remaining amount of ink reaches a certain level.
Furthermore, as a third reason, when a large-sized print sheet is used, if the remaining amount of ink is not detected precisely, it is possible that ink is used up during printing and printing up to that time becomes vain.
From the viewpoints of safety, economy, etc., described above, it is necessary to detect the remaining amount of ink accurately.
Known method of detecting the remaining amount of ink include (1) mechanical detection, (2) optical detection, (3) detection based on change in electrical resistance, (4) detection based on change in capacitance, and (5) detection based on count of discharged amount.
Examples of (3) detection based on change in electrical resistance include (1) Japanese Unexamined Patent Application Publication No. 6-226990 (Patent Document 1), (2) Japanese Patent Publication No. 2772015 (Patent Document 2), (3) Japanese Patent Publication No. 2798948 (Patent Document 3), and (4) Japanese Unexamined Patent Application Publication No. 11-179936 (Patent Document 4).
Of the examples of detection based on change in electrical resistance, according to the methods disclosed in Patent Documents 1to 3, a pair of electrodes is provided in a liquid, and a current is fed to the electrodes from a DC power source via a resistor having a high value of resistance. The voltage applied to the pair of electrodes changes depending on the presence or absence of the liquid between the pair of electrodes. According to the method disclosed in Patent Document 4, an alternating current is used for detection of a liquid.
The related art described above, however, has had the following problems.
First, when a direct current flows through a liquid as in the art disclosed in Patent Documents 1 to 3, an electrolysis occurs depending on the type of the electrodes and the components of the liquid. Thus, the surfaces of the electrodes are likely to change, and metallic ions are eluted into the liquid, possibly causing change in the characteristics of the liquid (ink). Furthermore, according to the methods that use a direct current, as will be described later in relation to embodiments of the present invention, the impedance of the circuit system tends to be high, which makes a detection at high speed difficult.
According to the art disclosed in Patent Document 3, in order to overcome this drawback, the direction of a current that flows through electrodes is reversed at a cycle of measurement period. According to this method, however, a direct current is used for measurement itself, and ions generated by the measurement with the direct current are eliminated by a flow of a DC current in the reverse direction for the same length of time. Therefore, the speed of measurement is slow.
According to the art disclosed in Patent Document 4, since an alternating current is used, the problem of the electrolysis does not occur. However, liquid is detected in an analog manner, i.e., by detecting change in the amount of the liquid based on change in capacitance. Thus, levels detected are unstable, and results of detection are not reliable.